The present invention relates to methods for rapid prototyping metal components, and in particular to casting of hollow metal components in a secondary mold having an integral core and shell, the secondary mold being formed from a rapid prototyped primary mold.
Components having complex geometry, such as components having internal passages and voids therein, are difficult to cast using conventional methods; tooling for such parts is both expensive and time consuming, for example, requiring a lead-time of at least four months. This situation is exacerbated by the nature of conventional molds comprising a shell and one or more separately formed cores, wherein the core(s) are prone to shift during casting, leading to low casting tolerances and low casting efficiency (yield). Examples of components having complex geometry and which are difficult to cast using conventional methods, include hollow airfoils for gas turbine engines, and in particular relatively small, double-walled airfoils. Examples of such airfoils for gas turbine engines include rotor blades and stator vanes of both turbine, and compressor sections, or any parts that need internal cooling.
In prior art methods for casting hollow parts, a ceramic core and shell are produced separately: a ceramic core (for providing a hollow of the part) is assembled into a wax tool that will provide the external shape of the part, the core is encased in wax, a ceramic shell is formed around the wax pattern, and the wax is removed to form a ceramic mold in which a metal part may be cast. Such prior art methods are not only expensive and have long lead-times, but have the disadvantage of low casting yield, for example, due to lack of reliable registration between the core and shell allowing movement of the core relative to the shell during filling the mold with molten metal. In the case of hollow airfoils, another disadvantage of such prior art methods is that shaped film holes must be formed by an expensive, separate step after forming the cast part, for example, by electron discharge machining (EDM) or laser drilling.
Development time and cost for airfoils, such as turbine blades, are magnified because such components generally require several iterations, sometimes while the part is in production. To meet durability requirements, turbine blades are often designed with increased thickness and with increased cooling airflow capability in an attempt to compensate for poor casting tolerance, resulting in decreased engine efficiency and lower engine thrust. Improved methods for casting turbine blades will enable propulsion systems with greater range and greater durability, while providing improved airfoil cooling efficiency and greater dimensional stability.
U.S. Pat. No. 6,375,880 to Cooper et al. discloses a method for making molded parts employing shape deposition manufacturing of a layered structure having support segments and mold segments. After removal of the support segments, part material is cast in the mold, and the mold is removed to provide a molded part.
U.S. Pat. No. 6,152,211 to Klug et al. discloses forming a green product and a fired ceramic article by pouring or injecting a ceramic slurry into a die. The ceramic article may be used as a shell mold or core for investment casting of eutectic and superalloy materials. Neither Cooper et al. nor Klug et al. disclose a method for rapid prototyping metal components.
As can be seen, there is a need for methods that allow the rapid prototyping and accurate casting of metal components having complex geometry, such as hollow airfoils for turbomachinery.